1. Field of the Invention
The present invention relates to a method of evaluating the adsorption of contaminants on a solid surface, particularly, to a method adapted for measuring the amount of adsorption of contaminants floating in a cleansing room, such as a clean room, on a workpiece, such as a silicon wafer, and for evaluating the cleanliness within the cleansing room and the contamination of the workpiece itself or on the cut solid surface.
2. Description of the Related Art
In accordance with progress in the degree of integration of the semiconductor device, the semiconductor device has been miniaturized and a multi-layered structure has come to be widely employed. In this connection, the manufacturing process of the semiconductor device has come to be affected by various impurities. It is known to the art that the manufacturing process of the semiconductor device is more or less affected by traces of, i.e., about picograms (10xe2x88x9212 g) of, all the atoms and molecules except silicon. Particularly, many kinds of organic compounds represented by carbon compounds are used in the manufacturing process of the semiconductor device. For example, a photoresist material, which is a typical organic compound, is used twenty times or more in the manufacturing process of the semiconductor device. Also, many materials generating organic compounds are used in a cleansing room, such as a clean room. For example, a plastic material is used in the duct, piping, wall material, and floor material. What should be noted is that a large amount of an organic compound, such as dibutyl phthalic acid (DBP) or dioctyl phthalic acid (DOP), is contained as a plasticizer in the plastic material. Such an organic compound contained in the plasticizer, which provides a contaminant, is evaporated from the duct, piping, wall material, floor material, etc. even under a low pressure and room temperature so as to float within the clean room. Such a floating contaminant is adsorbed on the workpiece, such as a silicon wafer, so as to contaminate the workpiece. Other contaminants, such as silicon, quartz, glass, and a metal material, also float within the clean room.
The methods for measuring the contamination includes, for example, a method for evaluating the contaminant, such as an organic compound, which is adsorbed on the wafer surface. In this method, the wafer itself is left to stand within an atmosphere so as to be measured by a wafer heating desorption type gas chromatograph for the mass analysis.
The conventional evaluation method referred to above will now be described with reference to FIG. 11. As shown in the drawing, a silicon wafer transferred and stored in a plastic container is taken out of the plastic container (steps 1 and 2), and is heated in a heating device for cleansing the silicon wafer so as to remove the contaminant from the silicon wafer (step 3), thereby cleansing the silicon wafer. Alternatively, it is possible to cleanse the silicon wafer by using a heating-desorbing device of a silicon wafer. In this case, however, the number of analyses and measurements performed in a unit time is reduced by half. The cleansed silicon wafer is housed again in the plastic container and is transferred to a measuring site within a chamber, such as a clean room (step 4). The silicon wafer is taken out of the plastic container at the measuring site and is left to stand for a predetermined time (one to two hundred hours) in the measuring site (step 5). The silicon wafer having an organic compound adsorbed thereon is housed again in the plastic container so as to be transferred to the point of a measuring device. Then, the silicon wafer is taken out of the plastic container and, is set in a heating-desorbing device for a silicon wafer so as to be heated at 300xc2x0 C. for thirty minutes (steps 6 and 7).
The organic compound generated from the heated silicon wafer is caught by an absorption tube (trade name of Tenax Absorption Tube) containing an activated carbon (step 8). Then, the adsorption tube is subjected to a heat treatment at 280xc2x0 C. for three hours by using an adsorption tube purging apparatus (trade name of Tenax adsorption tube purging apparatus), followed by washing the adsorption tube (step 9). Since the adsorption tube set in the heating-desorbing apparatus for the silicon wafer is heated, the organic compound is liberated (step 10). Further, the adsorption tube is mounted to a gas chromatograph for the mass analysis for the measurement of the material and mass of the liberated organic compound (step 11). It is evaluated whether or not the measured point is contaminated with the organic compound based on the result of the measurement (step 12).
Also known is a method in which a silicon powder is put in an adsorption tube and the air is sucked from, for example, a clean room into the adsorption tube for performing a desired measurement.
However, in the conventional method described above, in which a silicon wafer is used and the organic compound adsorbed on the wafer surface is evaluated, it is necessary to use first a heating apparatus for cleansing the silicon wafer and a heating-desorbing apparatus for the silicon wafer for liberating the organic compound adsorbed on the silicon wafer. The heating apparatus for cleansing the silicon wafer and the heating-desorbing apparatus for the silicon wafer give rise to problems. Specifically, these apparatuses are bulky, require a large mounting area, and are costly. Also, required is a Tenax adsorption tube.
It should also be noted that the cleansed silicon wafer and the silicon wafer having a contaminant, such as an organic compound adsorbed thereon, are housed in the same plastic container for the transfer and storage. As a result, the contaminant, such as an organic compound, is evaporated from the plastic container and is adsorbed on the silicon wafer so as to lower the measuring accuracy. Further, since the silicon wafer whose contaminant has been measured is stored in the plastic container for the transfer and storage, making it impossible to store the silicon wafer for a long time. It follows that the measuring site is limited to locations in the vicinity of the heating apparatus for cleansing the silicon wafer and the heating-desorbing apparatus for the silicon wafer. As a result, it is necessary to newly install the bulky and costly heating apparatus for cleansing the silicon wafer and heating-desorbing apparatus for the silicon wafer in the case where the measuring site is located far away, e.g., where the measuring site is located overseas. An additional problem to be noted is that the use of the heating apparatus for cleansing the silicon wafer and the heating-desorbing apparatus for the silicon wafer leads to an increase in the number of measuring steps, making it difficult to carry out the required evaluation promptly.
Also, the evaluating concentration of the contaminant, e.g., dioctyl phthalic acid (DOP), adsorbed on the silicon wafer is 0.2 nanogram (ng)/cm2. On the other hand, the gas chromatograph mass analyzing apparatus for detecting the contaminant has a high sensitivity and is capable of detecting DOP to an absolute concentration of 0.1 ng. However, in the case of using a silicon wafer, the silicon wafer has a surface area of 314 cm2 on one surface because the silicon wafer has a diameter of 200 mm. Therefore, the concentration corresponding to the detectable sensitivity noted above is 0.1 ng/314 cm2=0.00032 ng/cm2. Naturally, the sensitivity is excessively high, compared with the required sensitivity of 0.2 ng/cm2.
Also known is a method in which a silicon powder or pellets are put in an adsorption tube, and air within, for example, a clean room is sucked into the adsorption tube for carrying out the required measurement. In this method, however, the silicon powder or pellets put in the adsorption tube are caused to overlap with each other, with the result that the adsorption of the organic material on the silicon wafer is deviated. Clearly, this method is not adapted for the evaluation of the organic material adsorbed on the silicon wafer.
The present invention, which has been achieved in an effort to overcome the above-noted problems inherent in the prior art, relates to a method of evaluating the adsorption of contaminants on a solid surface, and is intended to provide a method of evaluating the adsorption of contaminants on a solid surface, which permits accurate measurement and evaluation by preparing, particularly, a test piece having a small surface area so as to simplify the handling of the pretreatment, evaluation, transfer and storage, and using the wafer itself while making it unnecessary to use bulky apparatuses required in the prior art.
According to the present invention, which is intended to achieve the above-noted object, there is provided a method of evaluating the adsorption of contaminants on a solid surface, comprising the steps of preparing a test piece of a material equal to the material of a workpiece, said test piece being sized to permit the test piece to be housed in an adsorption tube container that is mounted to a mass analyzing apparatus; leaving said test piece to stand in a measuring site under a test atmosphere for a predetermined time, followed by recovering the test piece and subsequently housing the recovered test piece in said adsorption tube container; and mounting the adsorption tube container housing said test piece to said mass analyzing apparatus so as to measure the contaminant material and the mass of the contaminant, thereby evaluating the degree of contamination of the measured point.
In the present invention, it is possible to evaluate the adsorption of a contaminant on a solid surface by using as the workpiece material silicon, quartz, glass, or a metallic material, such as stainless steel, aluminum, copper, brass, or nickel silver. It is also possible to use a silicon single plate as the test piece and to use any of silicon, quartz, or a glass material for forming the adsorption tube container housing said test piece.
In the present invention of the construction described above, a test piece of a silicon single plate is prepared by cutting away a part of a silicon wafer. After the test piece is washed, the washed test piece is housed in a small adsorption tube made of a washed quartz or glass so as to be transferred to a measuring site, such as a clean room. In this step, the washed test piece is housed in an adsorption tube container that can be mounted directly to the mass analyzing apparatus of a gas chromatograph. As a result, the present invention is free from the problem inherent in the prior art that the test piece is contaminated with contaminants, such as an organic compound. Also, since the test piece is shielded from the outer atmosphere so as to be put in a hermetically sealed state, it is possible to store the test piece for a long time. At the measuring site, the test piece is taken out of the small adsorption tube container and is left to stand at the measuring site for a predetermined time. The xe2x80x9cpredetermined timexe2x80x9d represents the time during which the contaminant reaches a predetermined surface saturation concentration, i.e., one to two hundred hours.
After the measurement, the test piece having the contaminant adsorbed thereon is housed in the small adsorption tube referred to above and transferred to the site where the measuring apparatus is installed. The small adsorption tube container housing the transferred test piece can be mounted as it is to a mass analyzing apparatus of a gas chromatograph. The mass analyzing apparatus of the gas chromatograph measures the contaminant material and the mass of the contaminant and evaluates based on the result of the measurement whether or not the measured point is contaminated with the contaminant.
Also, since the actual process wafer is cut, it is possible to evaluate the contaminant distribution within the wafer so as to contribute to the improvement in the defect analysis and yield.